I popped into Southern Aquatics And Pets in Poole, Dorset at lunchtime. They have some cages on sale at the moment - including the Ferplast Criceti 15 at £38 each! These are normally over £50 online, so that's quite a bargain. They seemed to have a several in stock, so if anyone's in the area and needs a bigger cage, get down there quick!
Here's the website with the address:
Southern Aquatics
They don't sell cages online, unfortunately.